Driving methods and circuit for bi-stable displays

ABSTRACT

The disclosure is directed toward driving methods and a driving circuit which are particularly suitable for bi-stable displays. In certain embodiments, methods provide the fastest and most pleasing appearance to the desired image while maintaining the optimal image quality over the life expectancy of an electrophoretic display device.

BENEFIT CLAIM REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 USC § 119(e) of provisionalapplication 60/942,585, filed Jun. 7, 2007, the entire contents of whichis hereby incorporated by reference for all purposes as if fully setforth herein.

TECHNICAL FIELD

The present disclosure relates to an electrophoretic display, and morespecifically, to driving approaches and circuits for an electrophoreticdisplay.

BACKGROUND

An electrophoretic display (EPD) is a non-emissive bi-stable outputdevice which utilizes the electrophoresis phenomenon of charged pigmentparticles suspended in a dielectric fluid to display graphics and/oralphanumeric characters. The display usually comprises two plates withelectrodes placed opposing each other. One of the electrodes is usuallytransparent. The dielectric fluid which includes a suspension ofelectrically charged pigment particles is enclosed between the twoplates. When a voltage potential is applied to the two electrodes, thepigment particles migrate toward the electrode having an opposite chargefrom the pigment particles, which allows viewing of either the color ofthe pigment particles or the color of the dielectric fluid.Alternatively, if the electrodes are applied the same polarity, thepigment particles may then migrate to the one having a higher or lowervoltage potential, depending on the charge polarity of the pigmentparticles. Further alternatively, the dielectric fluid may have a clearfluid and two types of colored particles which migrate to opposite sidesof the device when a voltage potential is applied.

There are several different types of EPDs, such as the conventional typeEPD, the microcapsule-based EPD or the EPD with electrophoretic cellsthat are formed from parallel line reservoirs. EPDs comprising closedcells formed from microcups filled with an electrophoretic fluid andsealed with a polymeric sealing layer are disclosed in U.S. Pat. No.6,930,818, entitled “Electrophoretic Display and Novel Process for ItsManufacture”, issued on Aug. 16, 2005 to the assignee hereof, the entirecontents of which is hereby incorporated herein by reference for allpurposes as if fully set forth herein.

Electrophoretic type displays are often used as an output display devicefor showing a sequence of different or repeating images formed frompixels of different colors. Because the history of voltage potentiallevels applied to generate the images is different for each pixel, thevoltage potential stress on each pixel of the display is typicallydifferent. These differences from pixel to pixel, in general, lead tolong term issues with image uniformity. Although attempts have been madepreviously to alleviate such problems with waveforms that have no DCbias or by use of clearing images to reduce non-uniformity, neither ofthese approaches provides a practical solution to such problems for thelong term.

SUMMARY OF THE DISCLOSURE

This disclosure is directed toward driving methods which areparticularly suitable for electrophoretic (bi-stable) displays and whichprovide the fastest and most pleasing appearance to a desired imagewhile maintaining optimal image quality over the life of anelectrophoretic display device.

A first embodiment is directed toward a driving method for a multi-pixelelectrophoretic display comprising a plurality of individual pixels,which method comprises applying voltage potentials across a displaymedium wherein the net magnitude of the voltage potentials applied,integrated over a period of time, are substantially equal for allpixels. The display medium for an electrophoretic display may be anelectrophoretic fluid.

A second embodiment is directed toward a driving method for amulti-pixel electrophoretic display comprising a plurality of individualpixels, which method comprises applying driving pulses to a given pixelwherein the total number of resets to a first color state and the totalnumber of resets to a second color state are substantially equal, forthe given pixel over a period of time. If there are more than two colorstates, substantially equal numbers of resets to each color state may beused, for a given pixel.

A third embodiment is directed toward a driving method for a multi-pixelelectrophoretic display comprising a plurality of individual pixels,which method comprises applying driving pulses to the pixels wherein thesums of resets to all states are substantially equal for all pixels. Ina more general case having more than two color states, the total numbersof resets to all color states are substantially equal for all pixels.

A fourth embodiment is directed toward a driving method for aelectrophoretic display comprising a plurality of individual pixels,which method comprises applying driving pulses to the pixels wherein thepixels are reset to a given color state after a certain number of thedriving pulses.

A fifth embodiment is directed toward a driving method for a multi-pixelelectrophoretic display comprising a plurality of individual pixels,which method comprises applying driving pulses to the pixels wherein thepixels have the substantially equal numbers of resets to each colorstate. As in the other embodiments listed above, this method can begeneralized to more than two color states.

A sixth embodiment is directed toward a driving method for a multi-pixelelectrophoretic display device, in which a corrective waveform isapplied to ensure global DC balance (i.e., the average voltage potentialapplied across the display is substantially zero when integrated over aperiod of time) or to correct any of the imbalance in the first, second,third, fourth or fifth embodiment of the disclosure as described above.The corrective waveform is applied without affecting or interfering withthe driving of individual pixels to intended images and may be appliedat a time when the electrophoretic display would not normally be in theprocess of being viewed by a viewer.

The driving methods of the present disclosure can be applied to driveelectrophoretic displays including, but not limited to, one timeapplications or multiple display images (i.e., burst mode displayapplication). They also could be used with many other display typeswhich potentially suffer from the same lifetime issues.

In a further embodiment, a bi-stable driving circuit is provided whichis suitable for implementing the various driving methods disclosedherein.

The whole content of each of the other documents referred to in thisapplication is also hereby incorporated by reference into thisapplication in its entirety for all purposes as if fully set forthherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a typical electrophoretic displaydevice.

FIG. 2 a and FIG. 2 b illustrate a one time display drivingimplementation.

FIG. 3 illustrates an alternative driving implementation for a one timedisplay.

FIG. 4 is a diagram which shows how multiple messages may be displayedin succession.

FIG. 5 a, FIG. 5 b, and FIG. 5 c illustrate a driving implementation formultiple messages.

FIG. 5 d illustrates extended waveforms for correction of DC imbalance.

FIG. 6 depicts exemplary corrective waveforms.

FIG. 7 depicts a flow diagram for implementing one or more ofembodiments.

FIG. 8 depicts an exemplary driving circuit suitable for implementationof the various embodiments disclosed herein.

DETAILED DESCRIPTION

FIG. 1 illustrates a typical array of electrophoretic display cells 10a, 10 b and 10 c in a multi-pixel display 100 which may be driven by thevarious driving implementations presented herein. In FIG. 1, theelectrophoretic display cells 10 a, 10 b, 10 c, on the front viewingside, are provided with a common electrode 11 (which is usuallytransparent). On the opposing side (i.e., the rear side) of theelectrophoretic display cells 10 a, 10 b and 10 c, a substrate (12)includes discrete electrodes 12 a, 12 b and 12 c, respectively. Each ofthe discrete electrodes 12 a, 12 b and 12 c defines an individual pixelof the multi-pixel electrophoretic display 100, in FIG. 1. However, inpractice, a plurality of display cells (as a pixel) may be associatedwith one discrete pixel electrode.

An electrophoretic fluid 13 is filled in each of the electrophoreticdisplay cells 10 a, 10 b, 10 c. The discrete electrodes 12 a, 12 b, 12 cmay be segmented in nature rather than pixellated, defining regions ofan image to be displayed rather than individual pixels. Therefore, whilethe term “pixel” or “pixels” is frequently used in this disclosure toillustrate driving implementations, the driving implementations are alsoapplicable to segmented displays.

Each of the electrophoretic display cells 10 a, 10 b, 10 c is surroundedby display cell walls 14. For ease of illustration of the methodsdescribed below, the electrophoretic fluid 13 is assumed to comprisewhite charged pigment particles 15 dispersed in a dark colorelectrophoretic fluid 13.

The white charged particles 15 may be positively charged so that theywill be drawn to a discrete pixel electrode 12 a, 12 b, 12 c or thecommon electrode 11, whichever is at an opposite voltage potential fromthat of white charged particles 15. If the same polarity is applied tothe discrete pixel electrode and the common electrode in a display cell,the positively charged pigment particles will then be drawn to theelectrode which has a lower voltage potential.

In another embodiment, the white charged pigment particles 15 may alsobe negatively charged.

Also, as would be apparent to a person having ordinary skill in the art,the white charged particles 15 could be replaced with charged particleswhich are dark in color and an electrophoretic fluid 13 that is light incolor so long as sufficient contrast is provided to be visuallydiscernable.

In a first embodiment, the electrophoretic display 100 could also bemade with a transparent or lightly colored electrophoretic fluid 13 andcharged particles 15 having two different colors carrying oppositeparticle charges, and/or having differing electro-kinetic properties.

The electrophoretic display cells 10 a, 10 b, 10 c may be of aconventional walled or partition type, a microencapsulted type or amicrocup type. In the microcup type, the electrophoretic display cells10 a, 10 b, 10 c may be sealed with a top sealing layer. There may alsobe an adhesive layer between the electrophoretic display cells 10 a, 10b, 10 c and the common electrode 11.

In one embodiment, a driving implementation for an electrophoreticdisplay 100 comprising pixels is disclosed. In this embodiment, varyingvoltage potentials are applied across the electrophoretic fluid 13 suchthat the net vector magnitudes of the voltage potentials applied to theindividual pixels 12 a, 12 b, 12 c, when integrated over a period oftime, are substantially equal for all pixels 12 a, 12 b, 12 c of theelectrophoretic display 100. In this embodiment, variations in the netvector magnitudes of the voltage potentials applied to the individualpixels 12 a, 12 b, 12 c when integrated over a period of time should bemaintained within a tolerance of about 20%. However, tighter tolerancesin the net vector magnitudes of the applied voltage potentials of lessthan about 10% provides improved image quality and possibly longerelectrophoretic display life. Ideally, tolerances in the net vectormagnitudes of the applied voltage potentials in a range of 0-2% providesthe greatest improvement in displayed image quality but may require morecostly electronics to maintain tolerances in this range.

In a second embodiment, a driving implementation for an electrophoreticdisplay 100 comprising pixels 12 a, 12 b, 12 c utilizes driving pulsesapplied to a given pixel 12 a, 12 b, or 12 c in order to maintain acumulative number of “resets” between a first and second color state forthe given pixel to be maintained substantially equal over a period oftime.

The term “reset” is defined as applying a driving voltage pulse to thegiven pixel to cause the given pixel to change from an original colorstate to a different color state or from an original color state to adifferent shade of the original color state. The reset may occur as partof the driving voltage pulse method to cause images to change in thecourse of normal pixel operation, for the reduction of flicker effectsor may be used to correct for “history effects” provided by the passiveand persistent display nature of electrophoretic type displays. Forcorrection of “history effects,” the reset may occur when theelectrophoretic display 100 is not in active use or idle. The drivingvoltage potential pulse is applied across the electrophoretic fluid 13.

Since there are many different ways in which a reset can beaccomplished, and since the different types of resets have differentimpacts on the uniformity and lifetime of a multi-pixel electrophoreticdisplay 100, only some of the reset scenarios may be implemented in themethods described herein; depending on the time required forimplementation and on the cost of implementation. The following tableillustrates different reset scenarios for the term “reset”:

TABLE 1 RESET SCENARIOS Scenario Reset to White Reset to Dark Scenario IDark to white white to dark Scenario II white to white dark to darkScenario III intermediate to white intermediate to dark Scenario IV darkto white white to dark white to white dark to dark Scenario V dark towhite white to dark intermediate to white intermediate to dark ScenarioVI white to white dark to dark intermediate to white intermediate todark Scenario VII dark to white white to dark white to white dark todark intermediate to white intermediate to dark

The term “intermediate” color state, in the context of the presentdisclosure, is a mid-tone color between a first color state and a secondcolor state or a composite color of the first and second color states.For ease of illustration and understanding, it is assumed in the aboveTable 1 that the first and second color states are white and dark.However, it is understood that in a two color display system, the twocolors may be any two colors so long as they provide sufficient contrastto be differentiated by visual observation.

In the driving implementation discussed above, a pixel 12 a, 12 b, or 12c may have N¹ number of resets to the white state and N² number ofresets to the dark state where the number N¹ and N² are substantiallyequal.

However, depending on the reset scenario selected, the resets may becounted differently. For example, if Reset Scenario I is selected, onlythe “dark to white” and “white to dark” are counted and, in other words,a pixel has N¹ switches from “dark to white” and N² switches from “whiteto dark”.

Alternately, if Reset Scenario IV is selected, the reset to white willinclude not only “dark to white” but also “white to white” and the resetto dark will include not only “white to dark” but also “dark to dark”and, in this case, the total number of resets from “dark to white” and“white to white” would be N¹ and the total number of resets from “whiteto dark” and “dark to dark” would be N². As is apparent, the term“reset” may be any one of the possible reset scenarios as described inTable 1, which are applicable to all driving implementations describedin the present disclosure.

A third embodiment is directed toward a driving implementation for anelectrophoretic display 100 comprising pixels 12 a, 12 b, 12 c. In thisembodiment, driving pulses are applied to the pixels 12 a, 12 b, 12 cwhere the sums of reset to all states are substantially equal, for allpixels. For example, in this driving implementation, a given pixel mayhave N³ number of total resets to a first color state and a second colorstate, and where the remaining pixels also have a number of total resetsto the two color states which number is substantially equal to N³.Furthermore, in this embodiment, the numbers of resets to a particularcolor state may be the same or different among various pixels, althoughthe cumulative number of color resets is substantially the same. Forexample, a first pixel may be driven to the first color state 60 timesand to the second color state 40 times while a second pixel may bedriven to the first color state 70 times and to the second color state30 times. Both the first and second pixels are driven to alternate colorstates 100 times but not necessarily to the first and second colorstates equally.

In a fourth embodiment, a driving implementation for a electrophoreticdisplay 100 comprising pixels 12 a, 12 b, 12 c, is provided where thepixels are reset to a pre-determined color state after a certain numberof driving pulses have been applied to the pixels without regard to anyparticular pixel. For example, a reset to each pixel's original color isprovided after 10,000 driving pulses have occurred. Alternately, ratherthan counting the number of driving pulses, all pixels may be driven toa predetermined color state based on a predetermined amount of operatingtime. In this alternate embodiment, all of the pixels may not have beenapplied substantially equal numbers of driving pulses before they aredriven to the predetermined reset state.

In another alternate embodiment, each pixel is reset to a predeterminedcolor state when a predetermined number of driving pulses have beenreceived. However, since the operation of individual pixels varies, notall pixels will be driven to the reset color state at about the samepoint in time.

In a fifth embodiment, a driving implementation for a electrophoreticdisplay 100 comprising pixels 12 a, 12 b, 12 c is provided where thepixels are voltage potential driven to have substantially equal numbersof resets to each color state. For example, a given pixel may have N⁴number of resets to a first color state and N⁵ number of resets to asecond color state; likewise, in this embodiment, the remaining pixelsalso have a number of resets substantially equal to the first and secondcolor states of N4 and N⁵, respectively. As is apparent in this fifthembodiment, the pixels are voltage pulse driven such that the number ofresets to the first and second color states are substantially equal.

For example, if Reset Scenario V is selected, all pixels are voltagepulse driven to have N⁴ resets to the white state (including “dark towhite” and “intermediate to white”) and N⁵ resets to the dark state(including “white to dark” and “intermediate to dark”). In a furtherexample, if Reset Scenario VII is selected, all pixels are voltage pulsedriven to have N⁴ resets to the white state (including “dark to white”,“intermediate to white” and “white to white”) and N⁵ resets to the darkstate (including “white to dark”, “intermediate to dark” and “dark todark”). In all of these examples, N⁴ is substantially equal to N⁵.

In this and other embodiments, variation in the number of resets isintended to be maintained within a tolerance of about 20%. However,tighter tolerances in the number of resets of less than about 10%provides improved image quality and possibly longer electrophoreticdisplay life. Ideally, tolerances in the number of resets in a range of0-2% provides the greatest improvement in displayed image quality but asdiscussed previously may be more costly to implement.

In a sixth embodiment, a corrective waveform is applied to the commonelectrode 11 and the individual pixel 12 a, 12 b, 12 c electrodes toensure global DC balance of the electrophoretic fluid 13 contained ineach electrophoretic cell 10 a, 10 b, 10 c. The corrective waveformattempts to normalize the voltage potentials applied to theelectrophoretic fluid 13 so that substantially a net zero volts existwhen integrated over a period of time. The global DC balance isconsidered to be sufficiently obtained if an imbalance of less than 90volt·sec (i.e., 0 to about 90 volt·sec) is accumulated over a period ofat least about 60 seconds. Improved results are realized if theimbalance of less than 90 volt·sec is achieved over a range of about 60minutes to about 60 hours. The application of the corrective waveformassists in maintaining uniformity of the electrophoretic fluid 13 amongall of the electrophoretic cells 10 a, 10 b, 10 c of the multi-pixelelectrophoretic display 100. The corrective waveform may also be appliedin addition to any of the pixel reset scenarios discussed above in thefirst, second, third, fourth or fifth embodiment. The correctivewaveform is typically applied at a later time so that it does notinterfere with the driving of pixels to intended images. The global DCbalance and other types of balance as described in the presentdisclosure are important for maintaining maximum long term contrast andfreedom from residual images.

In this embodiment of the disclosure, programmable circuits are used tocorrect for the DC imbalance at periodic intervals utilizing acorrective equalizing waveform. For example, a microcontroller 800 (FIG.8) may be used to keep track of the level of DC imbalance, and correctfor imbalances on a regular basis. The microcontroller 800 may comprisea memory element 802 which records the cumulative number of voltagepulses applied to a given pixel, or a number of resets to a given colorstate for each pixel, over a period of time. At some periodic interval(i.e., once per predetermined time period, or some time after a sequenceof driving voltage pulse waveforms), a separate corrective waveform mayalso be applied which substantially compensates for DC imbalancesrecorded in the memory 802. A more detailed discussion of themicrocontroller 800 and associated circuitry is provided in FIG. 8below.

The corrective waveform may be accomplished either at a separate timewhen the electrophoretic display 100 would be expected to be idle orwhen it would otherwise not interfere with normal driving of intendedpixels (i.e., during normal display), or as an extension of anotherpredetermined waveform so as to not be visually discernable. Forexample, a corrective waveform is provided at a duration or rate notdiscernable to an observer. Several embodiments of this correctivedriving implementation can be envisioned, depending on the intendedapplications. A few of these are described below. However, a personhaving ordinary skill in the art will appreciate that many variations ofthe methods disclosed below may be provided.

In a first embodiment, a corrective waveform is used and imbalances inpixels 12 a, 12 b, 12 c may be corrected at a time when anelectrophoretic display 100 is not in operation, for example, in themiddle of the night or at a predetermined time when the electrophoreticdisplay 100 is not expected to be in use. Although many applications areperceived for this method of achieving the balance, a smartcard havingan integrated electrophoretic display 100 or other similar securitytoken devices are examples which may benefit from a corrective waveform.For example, when a smartcard is used, a user wants to review thedisplayed information as quickly and easily as possible, but followinguse, the smartcard is then typically disposed in the user's wallet forthe majority of time, so that a corrective waveform applied at a latertime will rarely be observed by the user.

In a second embodiment, no corrective waveform is required. Instead, alonger driving voltage potential pulse is applied. This approach isparticularly useful if the longer driving voltage potential pulse is atthe end of a normal driving sequence so that there would be no visualimpact on the image displayed. The additional amount of time requiredfor the driving pulse is determined by a microcontroller 800 and shouldbe sufficiently long in order to compensate for the imbalance which havebeen stored in the memory 802 of the microcontroller 800 based on thedriving history or changes in color state of the pixels 12 a, 12 b, 12 c(FIG. 1)

An imbalance of too many white pixels may be corrected by applying alonger driving pulse when the white pixels are driven to the dark state,especially if the dark state occurs at the end of a normal drivingsequence. Such a corrective waveform extension can be used to correctfor DC imbalance or net vector magnitudes of applied voltage potentialsto the pixels 12 a, 12 b, 12 c as discussed above. In embodiments of thedisclosure involving equalization of the number of resets, the extendedcorrective waveform comprises a number of resets used to achieve thecorrection. This embodiment of the disclosure is demonstrated in Example5 below.

In a third embodiment of this corrective driving implementation, the DCimbalance may also be corrected with a color flash (i.e., driving allpixels to a predetermined color state, sometimes referred to as a “whiteflash,”) at the beginning of the next sequence of normal displaywaveforms. For normalizing the global DC balance, this will allow for azero time average DC bias and help to display cleaner images. However,this driving implementation may provide an undesirable initial displayflash at the time of initiation of the next sequence of waveforms.

The driving implementations of the present disclosure are applicable toa variety of electrophoretic displays. In an electrophoretic display 100with a traditional up-down switching mode, the charged pigment particles15 move in a vertical direction between the electrodes 11 and 12 a, 12b, 12 c as shown in FIG. 1, depending on the voltage potentials appliedto the electrode layers 11 and 12 a, 12 b, 12 c. If the electrophoreticdisplay fluid 13 comprises charged white particles 15 dispersed in adark color fluid, the images displayed by this electrophoretic display100 would be in white/dark colors.

The driving implementations of the present disclosure may also beapplied to an electrophoretic display with an in-plane switching mode.Examples of in-plane switching electrophoretic display are described inE. Kishi, et al., “5.1: Development of In-plane EPD”, Canon ResearchCenter, SID 00 Digest, pages 24-27 (2000); Sally A. Swanson, et al.(2000); “5.2: High Performance EPDs”, IBM Almaden Research Center, SID00 Digest, pages 29-31 (2000); and U.S. Pat. No. 6,885,495, entitled“Electrophoretic Display with In-plane Switching”, issued Apr. 26, 2005,to the assignee hereof, the entire contents of all the above documentsare incorporated by reference herein in their entirety as if fully setforth herein. A typical in-plane switching electrophoretic display mayalso exhibit two contrasting colors.

Furthermore, the driving implementations described herein may also beadapted to a electrophoretic display which is capable of displaying morethan two color states, such as a dual mode electrophoretic display asdescribed in U.S. Pat. No. 7,046,228, entitled “Electrophoretic Displaywith Dual Mode Switching,” issued on May 6, 2006 to the assignee hereof,the content of which is herein incorporated by reference in its entiretyfor all purposes as if fully set forth herein.

EXAMPLES

For ease of illustration and understanding of the various correctivewaveforms of the present disclosure, a set of drawings is provided inFIG. 2 to FIG. 7. With respect to FIG. 2 to FIG. 7, the electrophoreticdisplay 100 (FIG. 1) is assumed to be comprise white charged pigmentparticles 15 dispersed in a dark electrophoretic color fluid 13 and theparticles 15 are positively charged so that they will be drawn to adiscrete pixel electrode 12 a, 12 b, 12 c or the common electrode 11,whichever has an opposite polarity or at a lower voltage potential.

Example 1 One Time Display Implementation

In this example, some of the images would be displayed on theelectrophoretic display 100 only once. For one time displayimplementations, the displayed image on the electrophoretic display 100is to be turned off or cleared after a pre-determined display period,for example, a one time password used in a smartcard application. Afterthe onetime password is generated and displayed, the password imageshould be cleared for security reasons. In this implementation, theelectrophoretic display 100 will be driven to the dark state and thenwait for the next driving sequence.

FIG. 2 illustrates one of the onetime display driving embodiments. Inthis embodiment, the initial color state or the “off” state of theelectrophoretic display 100 is represented by the dark color state ofthe electrophoretic fluid 13 (display medium.) As depicted, the drivingimplementation has two phases, a driving phase and a clearing phase. Thedriving phase is shown in FIG. 2 a. The clearing phase, as shown in FIG.2 b, has two frames 201 and 202. The top waveform in FIG. 2 a shows thatno voltage potential is applied to the common electrode in the drivingphase. Waveform I in FIG. 2 a shows a voltage potential of +V is appliedto drive the white pixels from the dark state (i.e., “off state”) to thewhite (visible) state. Waveform II shows that no voltage potential isapplied so that the dark pixels remain in the dark state during thedriving phase.

In the clearing phase as shown in FIG. 2 b, no voltage potential isapplied in frame 201 and a voltage potential of +V is applied in frame202, to the common electrode 11 (FIG. 1) For the white pixels to becleared, initially no voltage potential is applied across the displaymedium 13 in frame 201 and the white pixels remain white in frame 201followed by a voltage potential of −V (shown as a net “0” V value) beingapplied across the display medium 13 in frame 202 which causes the whitepixels to revert to the dark state (the “off” state) in frame 202. Inthis approach the common is +V and the pixel is 0, and therefore the netvoltage potential is −V. For the dark pixels to be cleared (i.e., toremain dark in the dark state), a voltage potential of +V is appliedacross the display medium 13 in frame 201 which drives the dark pixelsto the white state in frame 201 and a voltage potential of −V (shown asa net “0” V value) is applied across the display medium 13 in frame 202which drives the dark pixels back to the dark “off” state in frame 202.Therefore at the end of the clearing phase, both the white and darkpixels are returned to the original dark “off” state. In the drivingimplementation of FIG. 2, when the duration of the driving phase of FIG.2 a is substantially equal to that of frame 202 shown in FIG. 2 b andthe durations of the frames 201 and 202 are also substantially equal, aglobal DC balance can be achieved. The driving implementation of FIG. 2also represents the first embodiment of the disclosure, that is, the netvector magnitudes of the voltage potentials applied, integrated over aperiod of time, are substantially equal for all pixels (i.e., white anddark), provided that when the duration of the driving phase issubstantially equal to that of frame 202 and the durations of the frames201 and 202 are also substantially equal.

The driving implementation of FIG. 2 also represents the secondembodiment of the disclosure, that is, the number of resets to the whitestate (D to W) is equal to the number of resets to the dark state (W toD), for each pixel. The driving implementation of FIG. 2 furtherrepresents the third embodiment of the disclosure, that is, the totalnumber of resets to the dark state and to the total number of resets towhite state are the same for both white and dark pixels (i.e., 2). Thedriving implementation of FIG. 2 further represents the fourthembodiment of the disclosure, that is, all pixels are reset to the darkstate after a series of driving pulses.

The driving implementation of FIG. 2 further represents the fifthembodiment of the disclosure as all pixels have the same number ofresets to the white state and the same number of resets to the darkstate.

Example 2 Alternative One Time Display Implementation

Experience has shown that if an electrophoretic display remains inactivefor an extended period of time, the performance of transitioning fromthe dark state to the white state or vice versa may become degraded, andthe dark state may have assumed a less than optimal charge value. FIG. 3illustrates an alternative driving phase to that in FIG. 2 to addressthis issue. As shown in the FIG. 3, the driving phase in thisalternative implementation has two driving frames, 301 and 302. For thecommon electrode 11 (FIG. 1) in this driving implementation, no voltagepotential is applied in driving frame 301 and a voltage potential of +Vis applied in driving frame 302. Waveform I drives pixels from the dark“off” state to the white state by applying across the display medium 13a voltage potential of +V in frame 301 and no voltage potential in frame302 and as a result, the pixels switch to the white state in frame 301and remain in the white state in frame 302. Waveform II, on the otherhand, keeps pixels in the dark state by applying across the displaymedium no voltage potential in frame 301 and a voltage potential of −V(shown as a net “0” V value) in frame 302 and in this case, the darkpixels remain dark in driving frame 301 and further driven to the darkstate in frame 302. The addition of the driving frame 302 has the effectof improved contrast ratio, especially if the electrophoretic displayhas undergone a prolonged period of inactivity. The clearing phase ofthis implementation is the same as that of FIG. 2 b.

The duration of driving frame 301 does not have to be equal to theduration of driving frame 302. However, in order to maintain the globalDC balance discussed above, the duration of frame 301 is generallymaintained substantially equal in duration to that of the frame 202.Accordingly, the duration sum of driving frame 302 and frame 202 (FIG.2) are substantially equal to the duration of frame 201.

Example 3 Multiple Message Display Implementation

An electrophoretic display may display multiple images sequentially. Themultiple messages may be shown in sequence within a short period of time(e.g., 1-2 minutes) and the final message may remain for a longer periodof time unless cleared or corrected. The multiple messages may bedisplayed one after another or the multiple messages may be a repeat oftwo or more messages, switching back and forth as driven by amicrocontroller 800 (FIG. 8)

FIG. 4 depicts an example as to how multiple messages may be displayedin succession. In the sequence as shown, the “idle” time betweenmessages is optional. The final message in the sequence may remain for aperiod of time, if needed. A corrective waveform may be applied betweenmessages (not shown) or after the second message has been displayed todrive the white pixels to the dark state and provide DC balancing asbriefly discussed above and discussed in more detail with respect toFIG. 5 below. To illustrate a clear example, FIG. 4 shows two messagesfollowed by a correction, but other embodiments may use three or moremessages.

FIG. 5 depicts one of the driving implementations for multiple messages.For exemplary purposes, FIG. 5 a, FIG. 5 b, and FIG. 5 c provide astring of three consecutive messages, First Message, Second Message andThird Message. Each of the messages is provided with a clearing phaseand a driving phase. For all three messages in this implementation, thecommon electrode 11 (FIG. 1) is always applied a voltage potential of +Vin the clearing phase and no voltage potential is applied in the drivingphase.

In FIG. 5 a (First Message), for Waveform I representing white pixels toremain in the white state, a voltage potential of −V (shown as a net “0”V value) is applied across the display medium 13 in the clearing phaseand a voltage potential of +V is applied across the display medium 13 inthe driving phase, and in this case, the white pixels are driven to thedark state in the clearing phase and then back to the white state in thedriving phase. For Waveform II representing white pixels to driven tothe dark state, a voltage potential of −V (shown as a net “0” V value)is applied across the display medium 13 in the clearing phase and novoltage potential is applied across the display medium 13 in the drivingphase, and as a result, the white pixels are driven to the dark state inthe clearing phase and remain in the dark state in the driving phase.For Waveform III representing dark pixels to be driven to the whitestate, no voltage potential is applied across the display medium 13 inthe clearing phase and a voltage potential of +V is applied across thedisplay medium 13 in the driving phase, and in this case, the darkpixels remain in the dark state in the clearing phase and are driven tothe white state in the driving phase. For Waveform IV representing darkpixels to remain in the dark state, a voltage potential of −V (shown asa net “0” V value) is applied across the display medium 13 in theclearing phase and no voltage potential is applied across the displaymedium in the driving phase, and as a result, the dark pixels remain inthe dark state in both the clearing and driving phases. The ThirdMessage (FIG. 5 c) has the same driving waveforms as the First Message(FIG. 5 a). However, the Second Message, between the First and ThirdMessages has different waveforms from 1 and IV.

In FIG. 5 b (Second Message), Waveforms II and III are the same as thoseof FIG. 5 a and FIG. 5 c. However, for Waveform I representing whitepixels to remain white, no voltage potential is applied across thedisplay medium 13 in either the clearing or driving phases, and in thiscase, the white pixels remain white in the clearing and driving phases.For Waveform IV representing dark pixels to remain in the dark state, novoltage potential is applied across the display medium 13 in both theclearing and driving phases, and as a result, the dark pixels remain inthe dark state in the clearing and driving phases.

The driving implementation as depicted in FIG. 5 has certain features.For example, no pixels need to be driven if there is no color statechange in the Second Message (see Waveforms I and IV of FIG. 5 b). Ifthere is a required change in the color state in pixels caused by theSecond Message, the pixels are driven to the desired color stateaccordingly. In the First and Third Messages, a white pixel remaining inthe white state is driven to the dark state first and back to the whitestate and a dark pixel remaining in the dark state is re-driven to thedark state first, to ensure refreshing of the dark pixels. Depending onthe implementation, an idle time may be provided between each of themessages. The idle time, as stated above, is optional.

The driving implementation for multiple messages as described in thisexample has many advantages. For example, only pixels having color statechange in consecutive messages are driven. Therefore, the image changemay occur at a high speed. In addition, the driving implementation alsoprovides refreshing of pixels to maintain good bistability. A correctivewaveform may be added at the end of the driving sequence to correct anyDC imbalances (see Examples 4 and 5 below) occurring from non-uniformpixel operation.

Example 4 Offline Corrective of Global DC Balance

In this example, the Waveforms I-IV described above for FIG. 5 a, FIG. 5b, and FIG. 5 c are used to illustrate the use of a post correctivewaveform. The driving implementation of Example 3 above provides a veryclean image switching sequence for displaying multiple messages;however, this implementation could generate a DC imbalance which if leftuncompensated, could cause image degradation in some circumstances.

Table 2 shows various combinations of driving scenarios for a string ofthree messages. According to Table 2, the waveforms of Example 3 (seeFIG. 5) may give a maximum imbalance, at the end of the entire sequence,of 1(−V), 0 or 1(+V), assuming that all the driving and clearingwaveform elements have the same duration (t₀).

TABLE 2 Driving Sequence for Three Consecutive Messages Balance CaseUtilization First Message Second Message Last Message Total # of Total #of Applied Applied Applied # of Total # of Driving Driving VoltageVoltage Voltage DC Driving Pulses Pulses Transition potential Transitionpotential Transition potential Offset Pulses to White to Dark W-W 0 W-W0 W-W 0 0 0 0 0 0 0 0 0 0 0 0 0 0 W-D −V 1(−V) 1 0 1 0 0 −V 1(−V) 1 0 10 W-D −V D-W +V 0 2 1 1 0 −V +V 0 2 1 1 0 −V D-D 0 1(−V) 1 0 1 0 −V 01(−V) 1 0 1 W-D −V D-W +V W-W 0 0 2 1 1 −V +V 0 0 2 1 1 −V +V W-D −V1(_V) 3 1 2 −V +V −V 1(−V) 3 1 2 −V D-D 0 D-W +V 0 2 1 1 −V 0 +V 0 2 1 1−V 0 D-D 0 1(−V) 1 0 1 −V 0 0 1(−V) 1 0 1 D-W +V W-W 0 W-W 0 1(+V) 1 1 0+V 0 0 1(+V) 1 1 0 +V 0 W-D −V 0 2 1 1 +V 0 −V 0 2 1 1 +V W-D −V D-W +V1(+V) 3 2 1 +V −V +V 1(+V) 3 2 1 +V −V D-D 0 0 2 1 1 +V −V 0 0 2 1 1 D-D−V D-W +V W-W 0 0 2 1 1 −V +V 0 0 2 1 1 −V +V W-D −V 1(−V) 3 1 2 −V +V−V 1(−V) 3 1 2 −V D-D 0 D-W +V 0 2 1 1 −V 0 +V 0 2 1 1 −V 0 D-D 0 1(−V)1 0 1 −V 0 0 1(−V) 1 0 1

FIG. 6 shows the waveforms for correcting the DC imbalance when thecorrective waveforms are initiated at some time after the end of thelast message set (Third Message), for example, after 30 seconds. Ifthere is no DC imbalance in the driving sequence for a given pixel, suchas that shown in the rows in Table 2 with zero DC offset, the correctiveWaveform 6 a (FIG. 6) may be applied which does not impact any currentlydisplayed images. If the desired end state is dark and there is animbalance of one dark pixel 1 (−V), the corrective Waveform 6 b may beapplied. If the desired end state is dark and there is an imbalance ofone white pixel 1(+V), Waveform 6 c may be applied. If the desired endstate is white and there is an imbalance of one white pixel 1 (+V), thecorrective Waveform 6 d may be applied. If the desired end state iswhite and there is an imbalance of one dark pixel 1 (−V), then Waveform6 e may be applied. The combined set of waveforms shown in FIG. 5 a,FIG. 5 b, FIG. 5 c and FIG. 6 will correct the DC imbalance.

When any of the corrective waveforms is applied, if for any reason,there is another message demand before, for example, the 30 secondinterval, that message demand would override the corrective waveform anddisplay the additional message, and after that second message iscomplete and another 30 seconds has expired, then one of appropriatecorrective waveforms is applied a sufficient number of times to correctfor the net imbalance achieved since the last correction. If theadditional message causes additional imbalances, for example, of 1(−V),the Waveform 6 b or 6 e may then need to be applied twice to correct theimbalance of 2(−V). The example only demonstrates a few possiblecorrective waveforms, which can be modified or extended in a wide numberof corrective waveforms to compensate for different levels of DCimbalance. In a similar manner, any of the imbalances in the firstthrough fifth embodiments of this disclosure may also be corrected.

Example 5

In another corrective waveform technique, rather than adding a separatecorrective waveform, the existing waveforms are extended to correct a DCimbalance which can be achieved in a way not visually discernable. Forexample, FIG. 5 d shows an extended version of the Third Message of FIG.5 c. In FIG. 5 d, a set of waveforms “Extension DD” is added between theoriginal clearing and driving phases and another set of waveforms“Extension WW” is added after the driving phase. In the extended phases,each waveform is presented with two options, shown as the solid anddotted lines. The dotted lines indicate that the voltage potentials forthe dark or white states have been extended in time to correct animbalance from previous messages. The solid lines indicate that awaveform in which no voltage potential difference is applied across thedisplay medium, so that no change in the image state occurs and novisible impact on the images displayed is observed, except that the timeof the waveforms for the Third Message is lengthened to allow dottedframes DD or WW. As is apparent from this waveform, not every pixel canbe corrected in this way. For example, in Waveforms II and IV, thepixels in the dark state cannot be corrected with extended Waveforms WW;and as a result, they cannot be balanced until subsequent waveforms areapplied in which a corrective opportunity occurs. The microcontroller800 (FIG. 8) simply keeps track of which pixels need to be corrected andadds the extra length of waveforms at an opportune time.

Numerous applications may utilize the above driving implementations inone form or another. Some examples include, without limitation,electronic books, personal digital assistants, mobile computers, mobilephones, cellular phones, digital cameras, electronic price tags, digitalclocks, smartcards, security tokens, electronic test equipment andelectronic papers.

The present techniques may be applied to a wide variety of theelectronic devices. The smartcard is one of many examples. The smartcardcan be used for any application requiring information to be displayedincluding, but not limited to, a stored value from an internal memory ofthe device, a generated password from the internal electronics of thedevice and a transferred value from an external device to the smartcard.

Referring to FIG. 7, a process flow chart is shown for implementing oneor more of the disclosed embodiments. The process is initiated at block700 and continues to block 705. At block 705, a microcontroller 800(FIG. 8) waits for a message to be received from the device circuit 815(FIG. 8). When a message is received at block 710 by the microcontroller800 from the device circuit 815, the message is output to theelectrophoretic display at block 715 by the microcontroller 800. Atblock 720, the microcontroller 800 records certain parameters associatedwith the driving pulses applied to the pixels needed to display themessage output at block 715.

At block 725, the microcontroller 800 determines whether another messageis to be output to the electrophoretic display 100 (FIG. 1) If anothermessage is to be output 725, the microcontroller 800 outputs the messageto the electrophoretic display 100 as before at block 715 and likewiserecords the certain parameters in memory 802 associated with the drivingpulses applied to the pixels needed to display the message of block 715at block 720.

At block 725, if another message is not pending for output, themicrocontroller 800 proceeds to block 730 to determine whether a cleardisplay timer has elapsed. If microcontroller 800 determines that theclear display timer has not elapsed, the microcontroller 800 waits foranother message to arrive as previously described for blocks 715, 720and 725. If the microcontroller 800 determines at block 730 that theclear display timer has elapsed, the microcontroller 800 sends theproper driving pulses to clear electrophoretic display 100 at block 735.In one embodiment, the clearing of electrophoretic display 100 at block735 also causes the microcontroller 800 at block 740 to reset the cleardisplay timer to restart timing for clearing the electrophoretic display100.

In one embodiment, the microcontroller 800 determines if a displaycorrection is required at block 745. The display correction at block 745may be provided to substantially equalize the number of times a drivingpulse is applied to individual pixels, the number of resets to aparticular color state for individual pixels, the number of resets totwo or more color states for the individual pixels and/or correction ofa relative DC imbalance among the individual pixels as described above.At block 745, if the microcontroller 800 determines that displaycorrection is not required, the microcontroller 800 returns to block 705to wait for a message 820 from the device circuit 815 as previouslydescribed.

At block 745, if the microcontroller 800 determines that displaycorrection is required, the microcontroller 800 proceeds to block 750which applies one or more of the above described display corrections tothe multi-pixel electrophoretic display 100 such as pixel drive pulsebalance 755 and/or DC balance 760.

In one embodiment, at block 750, once the display correction has beenapplied and completed, the microcontroller 800 returns to block 705 towait for a message 820 from the device circuit 815 as previouslydescribed.

Referring to FIG. 8, an exemplary block diagram of a microcontrollercircuit suitable for implementing the various embodiments described isshown. In one embodiment, a microcontroller 800 includes a memory 802and an internal clock 804. The microcontroller 800 may be of any commonprogrammable type such as an ASIC, FPGA, CPLD, LSIC, microprocessor,programmable logic gate circuit or similar intelligent devices. Themicrocontroller 800 is provided with a DC power source 810 typicallyfrom a battery. In one embodiment, the microcontroller 800 isoperatively coupled to a bi-stable driver controller 805.

The bi-stable driver controller 805 converts signals received from themicrocontroller 800 into voltage driving pulses which are supplied tothe bi-stable display 100 by connections 805 a, 805 b. In oneembodiment, the bi-stable controller provides 50 millisecond (ms) to 500ms electrical driving pulses to the bi-stable display 100. In oneembodiment, the multi-pulse voltage driving frames of 200 ms to 1500 msare provided by the bi-stable driver controller 805 to the bi-stabledisplay 100. In one embodiment, the microcontroller 800 and bi-stabledriver controller 805 are integrated into a single form factor. Forexample, a field programmable gate array (FPGA) coupled to the bi-stabledisplay 100 using bipolar op-amps.

In one embodiment, the bi-stable controller 805 typically includes aDC-DC converter 807 which is used to increase the voltage supplied fromthe DC power source 810 to about 30-40 VDC. The messages 820 receivedfrom the device circuit 815 cause microcontroller 800 to signal thebi-stable controller 805 to output the message 820 to the bi-stable(electrophoretic) display 100.

In one embodiment, the microcontroller 800 is provided with logicalinstructions to perform the display corrective implementations describedabove, including but not limited to, substantially equalizing the numberof times a driving pulse is applied to individual pixels of bi-stabledisplay 100, the number of resets to a particular color state forindividual pixels of bi-stable display 100, the number of resets to twoor more color states for the individual pixels of bi-stable display 100and/or correction of a relative DC imbalance among the individual pixelsof bi-stable display 100 as described above.

Although the foregoing disclosure has been described in some detail forpurposes of clarity of understanding, it will be apparent to a personhaving ordinary skill in that art that certain changes and modificationsmay be practiced within the scope of the appended claims. It should benoted that there are many alternative ways of implementing both theprocess and apparatus of the improved driving scheme for anelectrophoretic display, and for many other types of displays including,but not limited to, liquid crystal, rotating ball, dielectrophoretic andelectrowetting types of displays. Accordingly, the present embodimentsare to be considered as exemplary and not restrictive, and the inventivefeatures are not to be limited to the details given herein, but may bemodified within the scope and equivalents of the appended claims.

1. A driving method for a multi-pixel electrophoretic displaycomprising: recording driving pulses applied to individual pixels of themulti-pixel electrophoretic display; and periodically applying acorrective waveform such that the driving pulses applied to all of theindividual pixels are substantially equalized over a period of time. 2.The driving method of claim 1 wherein the recording of the drivingpulses applied to the individual pixels is selected from the groupconsisting of: a number of times a driving pulse is applied to theindividual pixels, a number of resets to a particular color state forthe individual pixels, a number of resets to two or more color statesfor the individual pixels and a relative DC imbalance in applied drivingpulses among the individual pixels.
 3. The driving method of claim 1wherein the corrective waveform comprises applying a driving voltageacross the multi-pixel electrophoretic display such that a net vectormagnitude of the driving pulses over the period of time aresubstantially equalized for all the individual pixels.
 4. The drivingmethod of claim 2 wherein the individual pixels are consideredsubstantially equalized when differences in the number of driving pulsesamong all the individual pixels is less than or equal to 20% over theperiod of time.
 5. The driving method of claim 1 wherein the drivingpulses includes a duration in a range of about 50 milliseconds to 500milliseconds.
 6. The driving method of claim 2 further comprisingapplying the corrective waveform to at least those individual pixelsreceiving a lower number of driving pulses than other individual pixelssuch that all individual pixels are substantially equalized in thenumber of driving pulses over the period of time.
 7. The driving methodof claim 1 further comprising waiting for a predetermined amount of timebefore applying the corrective waveform to at least a portion of theindividual pixels.
 8. The driving method of claim 1 further comprisingdetermining whether a sufficient period of inactivity has elapsed forthe multi-pixel display before applying the corrective waveform to theindividual pixels.
 9. The driving method of claim 1 wherein thecorrective waveform includes a duration of less than 350 milliseconds.10. The driving method of claim 2 wherein a duration of the correctivewaveform is substantially equal to a cumulative duration of drivingpulses applied to a given pixel.
 11. The driving method of claim 2wherein the corrective waveform includes an opposite polarity from thedriving pulses applied to a given pixel.
 12. A driving method for amulti-pixel electrophoretic display comprising: applying driving pulsesto a given pixel; applying, over a period of time, a cumulative numberof resets to a first color state and a cumulative number of resets to asecond color state that are substantially equal over the period of time.13. The driving method of claim 12 further comprising applying acorrective waveform to the given pixel to correct for a DC imbalancerelative to other individual pixels of the multi-pixel electrophoreticdisplay.
 14. The driving method of claim 12 wherein the driving pulsesare provided in a multi-pulse driving frame having a duration in a rangeof about 200 milliseconds to 1500 milliseconds.
 15. A driving method fora multi-pixel electrophoretic display comprising: applying drivingpulses to individual pixels of the multi-pixel electrophoretic display;applying, over a period of time, a cumulative number of resets to allcolor states that is substantially equalized for all individual pixels.16. The driving method of claim 15 further comprising applying acorrective waveform for correcting DC imbalances among the individualpixels.
 17. The driving method of claim 15 method further comprisingapplying driving pulses to the individual pixels such that all of theindividual pixels are reset to a given color state after the individualpixels have been substantially equalized.
 18. A system for driving amulti-pixel electrophoretic display comprising: a microcontrolleroperatively coupled to the multi-pixel electrophoretic display, themicrocontroller including; a memory for recording parameters associatedwith driving pulses applied to individual pixels of the multi-pixelelectrophoretic display; and, a set of logical instructions which whenperformed by the microcontroller causes the microcontroller to;determine from the recorded parameters which individual pixels requireapplication of a corrective waveform; and, apply the corrective waveformto the individual pixels determined by the microcontroller.
 19. Thesystem according to claim 18 wherein the set of logical instructionsfurther comprises logical instructions which when performed by themicrocontroller causes the microcontroller to wait a predeterminedamount of time before applying the corrective waveform.
 20. The systemaccording to claim 18 wherein the corrective waveform substantiallyequalizes a number of driving pulses applied to all individual pixelsover a period of time.
 21. The system according to claim 18 wherein thecorrective waveform resets the individual pixels to a given color state.22. The system according to claim 18 wherein the corrective waveformresets the individual pixels to two or more color states.
 23. The systemaccording to claim 18 wherein the corrective waveform resets allindividual pixels such that a cumulative number of resets to all colorstates are substantially equalized.
 24. The system according to claim 18wherein the corrective waveform corrects for DC imbalances among theindividual pixels.
 25. The system according to claim 18 wherein thecorrective waveform substantially equalizes a net magnitude of drivingpulses for all the individual pixels over the period of time.
 26. Thesystem according to claim 18 wherein the corrective waveform is providedto obtain a DC imbalance of less than 90 volt·sec over a duration ofabout 60 minutes.